1. Field of the Invention
The present invention relates to a photodetection circuit and a confocal microscope having the circuit, and more specifically to a photodetection circuit for enlarging the photodetection dynamic range and improving the S/N ratio of height data, and a confocal microscope having the circuit.
2. Description of the Related Art
Conventionally, a confocal microscope applies dotted illumination to a specimen, converges transmitted light, reflected light, or fluorescence from the specimen on a confocal diaphragm, and detects by a photodetector the intensity of the light passing through the confocal diaphragm, thereby obtaining the surface information about the specimen. A scanning confocal microscope scans the surface of the specimen using dotted illumination in various methods, thereby obtain the surface information about the specimen in a wide range.
FIG. 1 shows the outline of the configuration of a conventional confocal microscope (scanning confocal laser microscope).
With the confocal microscope shown in FIG. 1, a laser beam output from a laser beam source 101 passes through a beam splitter 102, and enters a two-dimensional scanning mechanism 103. The two-dimensional scanning mechanism 103 has a first optical scanner 103a and a second optical scanner 103b, performs two-dimensional scanning using luminous flux, and leads it to an object lens 107. The luminous flux input to the object lens 107 becomes converging beam and scans the surface of a specimen 108.
The light reflected by the surface of the specimen 108 is introduced from the object lens 107 again to the beam splitter 102 through the two-dimensional scanning mechanism 103, then reflected by the beam splitter 102, and converges on a pinhole 110 by an image forming lens 109. The pinhole 110 cuts off the reflected light from the points other than the beam condensing point of the specimen 108, and a photodetector 111 detects the light only passing through the pinhole 110.
The specimen 108 is held on a specimen table 113. A stage 114 and the photodetector 111 is controlled by a computer 112.
The beam condensing position by the object lens 107 is in a position optically conjugate with the pinhole 110. When the specimen 108 is in the beam condensing position of the object lens 107, the reflected light from the specimen 108 converges on the pinhole 110 and passes through the pinhole 110. When the specimen 108 is displaced from the beam condensing position of the object lens 107, the reflected light from the specimen 108 does not converges on the pinhole 110, and does not pass through the 110.
FIG. 2 shows an I-Z curve indicating the relationship between the relative position (Z) of the object lens 107 to the specimen 108 and the output (I) of the photodetector 111.
As shown in FIG. 2, when the specimen 108 is in the beam condensing position Z0 of the object lens 107, the output of the photodetector 111 indicates a maximum value. As the relative position of the object lens 107 to the specimen 108 leaves from the position, the output of the photodetector 111 indicates a sudden decrease.
With the characteristic, if the two-dimensional scanning mechanism 103 performs two-dimensional scanning on the beam condensing point, and an image is generated by the output of the photodetector 111 in synchronization with the two-dimensional scanning mechanism 103, then an image of only a specific height portion of the specimen 108 is formed, and an image (confocal image) is obtained by optically slicing the specimen 108. Furthermore, the specimen 108 is discretely moved on the specimen 108 in the optical axis direction, the two-dimensional scanning mechanism 103 performs scanning in each position to obtain a confocal image, and the Z position of the stage 114 where the output of the photodetector 111 indicates the maximum value is detected, thereby obtain the height information about the specimen 108. Additionally, by overlaying and displaying the maximum value of the output of the photodetector 111 at each point of the specimen, an image can be obtained with all points of the image displayed in focus (extend image).
Thus, when there is a large difference in brightness with the confocal microscope, that is, when a specimen having a large brightness difference between a bright portion and a dark portion is observed, it is necessary to adjust the detection sensitivity not to reach saturation on the screen. In this case, the adjustment is to be made based on a bright portion, and the data of a dark point includes much noise. Therefore, generally, a bright portion is compressed using a logarithm amplifier (γ characteristic) while a dark portion is brightened to expand the photodetection dynamic range, thereby reducing the entire contrast (for example, Japanese Patent Laid-open Publication No. H7-212158). There also the technology of using the LUT (lookup table) to improve the visibility of a dark portion (for example, Japanese Patent Laid-open Publication No. H6-124192).